Kaposi's sarcoma-associated herpesvirus (KSHV), or human herpesvirus 8, is the causative agent of Kaposi's sarcoma (KS). In regions with high prevalence of both HIV and KSHV, such as sub-Saharan Africa, KS is a leading cancer. As with all herpesviruses KSHV can exist in either latent or lytic modes of replication, allowing the lifelong infection of its host. During latency the virus has limited gene expression and replicates along with the host cell without causing cytopathic effect. The lytic cycle includes the expression of the immediate early (IE), the early (E), and late (L) lytic genes, linear viral DMA replication, and ultimately the production of infectious virus. During latency the virus is quiescent, but under poorly understood conditions the virus spontaneously reactivates from latency to lytic replication. The control of the decision of latent or lytic replication is not well understood, but is a critical aspect of herpesvirus biology. The choice of latent or lytic replication has not been easy to study because most human herpesviruses do not readily establish latent infections in cell culture. Fortuitously, KSHV readily establishes a latent infection in a wide variety of cells in culture, providing an important model system for the study of latency, the examination of the lytic reactivation, and the factors involved in controlling latent or lytic replication. In order to identify viral factors impacting the decision of latent or lytic replication, we have taken advantage of the property of KSHV to establish latent infections to isolate a mutant virus that spontaneously enters lytic replication, with the formation of plaques and the production of infectious virus. This proposal will investigate the viral functions responsible for KSHV spontaneous lytic replication with the following specific aims: 1. Determination of the IE genes expressed, or activated, by X219. 2. Analyze the X219 genome to identify genetic differences compared to the parental virus. 3. Determine the genotype of X219 responsible for the phenotype of spontaneous lytic replication.